The present invention relates to thread measuring gages and, in particular, to positively expandable and retractable gages capable of determining the circularity of a threaded portion of an object.
The nature of the present gage best can be appreciated by considering the manner in which it is used to measure the circularity of threads formed on an end portion of, for example, a 5 inch 38 projectile. Such projectiles generally are in the form of cylindrical casings provided at their end portions with threads into which end fuse fittings or the like can be coupled. In fabricating these casings, great care must be exercised to assure, by means of bourrelet measurements, that the particular casing is truly cylindrical. Equal care also must be exercised to assure that the threaded end of the casing is precisely cylindrical to the extent that the axis of this threaded portion is concentric with that of the casing itself. Any inclination or departure from such a concentric relation results in a corresponding inclination of the coupled fuse relative to the axis of the cylindrical casing itself. Such deviations seriously affect the flight characteristics of the projectile. Gages or the like are employed to assure the precise circularity or, in other words, concentricity with the circularity of the missile itself.
Expandable gages are well known and frequently are formed with exterior threads to mate with the threads being measured. However, most of these gages simply are used to expand into a meshed contact and, as far as is known, they are not suitable for the present circularity measuring. Also, the mechanisms used to expand and contract the gages appear to be relatively complex and, in a number of instances, rather unreliable.
Another important factor to be considered is, obviously, that after a gage has been expanded into a meshed engagement or the like, it then must be contracted in size to permit its removal. Usually, gages of the present type employ some positive force, such as a cam, produce the expansion. However, when removal is desired, reliance then is placed upon some resilient means, such as a spring return, to free it for removal. Unfortunately, spring returns are not too reliable particularly when the meshed contact is so tight as to resist release. As a result, attempts to remove a gage may cause serious thread damage which requires costly and time-consuming repairs. This problem is of particular concern in mass production operations or, in other words, in fabrications in which the gaging operation is automated and remotely controlled rather than being a manually controlled operation. To resolve this problem, it is desirable to provide a gage which is not only positively expandable into thread contact but, in addition, is positively and forcefully retractable from it. The positive retraction force assures full contraction and avoids problems especially, as indicated, in automated operations. Also, as will become apparent, the construction and operation of the present gage provides a number of advantages apart from its capability for providing the concentricity measurements for 5 inches, 38 projectiles and the like.
In general, the present gage is a cam-operated expandable and retractable device in which a cam follower formed of a plurality of segments is provided with a pair of cam surfaces riding on a pair of cams which, in turn, are carried by a reciprocably driven shaft member. Reciprocation in one direction radially expands the follower segments while reciprocation in the opposite direction positively retracts them. The segments themselves are radially-extending members having circumferential edge portions provided with threads adapted to mesh with the threads to be measured. Concentricity measurements are achieved by permitting the shaft member and its housing to float during its final engagement between the segment threads and threads to be measured. Initially, the housing is held in the fixed position by a mechanical support. In this fixed position, it is disposed in an axially aligned relationship with the cylinder or projectile that carried the threads to be measured. As a predetermined point, the mechanical support is withdrawn to permit the floating action. When the thread circularity is faulty, the housing and its shaft tilt or incline away from its initially fixed position into a disposition in which it aligns with the axis of the threads being measured. Transducer means then can be used to determine the degree of the inclination which, of course, would be considered as an error in the thread fabrication.